This invention relates to coating of metallic articles and, more particularly, to diffusion aluminide type coating.
One of the more widely known uses of the well-known diffusion aluminide method for applying coatings is the application of an aluminide coating to the surface of turbine blading members of a gas turbine engine. As a result of this and other uses, a number of diffusion aluminiding methods and materials have been described in a wide variety of publications.
In general, the diffusion aluminide method involves cleaning the article surface to be coated and then placing such surface in contact with a halide vapor carrying the coating material in an inert or reducing atmosphere. The most commonly used method is the pack diffusion method in which the article is immersed or packed in a powdered mixture including a metallic powder which acts as the source of the coating material and a halide salt activator which reacts with the metallic powder to provide the coating metal halide vapor. One example of this method is described in U.S. Pat. No. 3,667,985, issued June 6, 1972. Another method, one example of which is described in U.S. Pat. No. 3,598,638, issued Aug. 10, 1971, involves contacting a surface to be coated only with a halide vapor of the coating metal rather than the metal itself.
In the manufacture of such coated articles, it frequently becomes necessary to repair such a coating, for example, as a result of the article having been operated in a gas turbine engine. In other instances, coating repairs are required during initial manufacture or on complete overhaul as a result of reworking procedures conducted after coating.
During initial manufacture of an article, the commonly used aluminide coating methods, which generally require a relatively high temperature, can be coordinated with joining operations such as brazing so as not to affect detrimentally the completely fabricated article. However, during repair or overhaul of such a brazed article, care must be taken that the repair coating must maintain the quality and integrity of the brazed joint.